Riner was a member of the Levallois Sporting Club in Levallois-Perret, France and is coached by Christian Chaumont and Benoît Campargue. He won the World and European junior titles in 2006. In 2007, he won a gold medal at the European Judo Championships in Belgrade, Serbia, on the day after his eighteenth birthday. At the 2007 World Judo Championships in Rio de Janeiro, Brazil, he became the youngest ever senior world champion when he won the heavyweight (+100 kg) event, defeating the 2000 Olympic gold medallist, Kosei Inoue of Japan, in the semi-final.
In his career, Riner was only defeated eight times in elite international championships. He lost to Brayson and Tölzer in 2006, to Bianchessi and Rybak in 2007 and to Muneta and Grim Vuijsters in 2008. He lost to Abdullo Tangriev in the third round of the 2008 Summer Olympics, before obtaining the bronze medal, and on 13 September 2010 he lost the openweight title at the 2010 World Judo Championships in Tokyo to Daiki Kamikawa, his last defeat before a series of 154 victories. After almost 10 years, he lost in the third round of the Paris Grand Slam against world number 2 Kokoro Kageura.